The present invention relates to a hydraulic control system for a continuously variable belt-drive automatic transmission for a motor vehicle having a torque converter with a lockup clutch, and more particularly to a control system for controlling lockup timing of the clutch.
A known control system for the continuously variable belt-drive transmission comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary a running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a secondary pressure control valve and a primary pressure control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The system has an electronic control system for controlling valves of the hydraulic circuit. Transmitting torque is accurately calculated for properly controlling secondary pressure regulated by the secondary pressure control valve to a necessary minimum value. The primary pressure is controlled by the primary pressure control valve to improve response of the transmission in accordance with the engine operating conditions. The electronic control system further controls various systems such as a failsafe system, an antilock brake system(ABS) and a lockup clutch of a torque converter.
In a continuously variable transmission(CVT) provided with the torque converter having the lockup clutch, when the lockup clutch is disengaged, engine output torque is multiplied by the operation of the torque converter. When the lockup clutch is engaged, the engine torque is directly transmitted to the automatic transmission. Therefore, it is preferable to control the secondary pressure in the transmission in accordance with the state of the lockup clutch.
Japanese Patent Application Laid-Open 1-206154 discloses a control system wherein a desired secondary pressure is calculated in accordance with a torque multiplication rate based on a speed ratio between speeds of an input member and an output member of the lockup clutch.
However, when the lockup clutch is operated simultaneously with timing of the increase or the decrease of the secondary pressure, various problems occur due to the response delay in a hydraulic circuit. One of the problems is as follow: at acceleration of the vehicle, increasing the secondary pressure is delayed even though the lockup clutch is already disengaged and the torque converter is operated. To the contrary, there may occur that the secondary pressure decreases before the clutch is completely engaged. Consequently, gripping force of the pulleys exerted on the belt decreases so that the belt slips, thereby decreasing durability of the belt and causing an abnormal shock. Therefore, it is desirable to adjust the timing for operating the lockup clutch and for controlling the secondary pressure so as to reliably increase the secondary pressure in a torque converter operational zone.
Japanese Patent Application Laid-Open 63-28740 discloses a system for controlling the timing of the continuously variable transmission(CVT). In the system, the transmission has a starting clutch and a direct connecting clutch bypassing the transmission. When engine speed exceeds a first reference speed, the direct connecting clutch is engaged to transmit the output power of the engine to driving wheels of the vehicle without passing through the transmission. When the engine speed decreases below a second reference speed which is lower than the first reference speed, the direct connecting clutch is disengaged, whereby the hunting of the system is prevented.
However, such a system having two clutches can not be applied to the CVT with the torque converter.